Approximately 50% of rice husk is now recycled into various by-products and while a significant number, there still exist unused. A special rice husk combustion unit generates steam and the rice husk ash is separated in hoppers. It is well known that rice husk content a high concentration of silica, generally more than 22%, and is unusually high in ash, which is more than 90% wt. Silica. This makes difficulties to use rice husk in combustion process in order to generate heat efficiently. Rice husk is also used as a fuel for boiling brine to produce NaCl salt, but its residue is costly to dispose of. The rice husk ash and rice husk blended with a Portland cement used to improve the quality of brine and bitters. The non-crystalline silica and high specific surface area of the rice husk ash are responsible for its high pozzolanic reactivity and has been used in lime-pozzolana mixes and could be a suitable partly replacement for Portland cement [1]. The rice husk can be converted to a useful form of energy. For example, the rice husk pyrolysis has been conducted in a fixed bed reactor [2]. Pyrolysis experiments were performed at temperature between 400 and 600 C. The goal was to obtain the highest liquid yield of 31.78% wt. at pyrolysis temperature 500 C and particle size 1.18 to 1.80 mm. Combustion and gasification of rice husk in fluidized bed reactors [3] is inefficient or unsuitable for energy conversion due to high ash content, low bulk density, poor flow characteristics and low ash melting point. Typically under the best operating conditions the gasification efficiency is around 65%. Few different methods exist for preparation of powdered activated carbon from rice husk [4, 5]. A drop-tube/fixed-bed reactor [4] experimental data show that rice husk containing a relatively high lignin content, had the lowest pyrolysis rate. Ash content was about 17.9% wt. Open core throat-less batch fed rice husk gasifier reactors was study for gasification efficiency and scaling factors [5].The parameters of the process were optimized specifically for this type of reactor. Most of the plasma gasification systems are based on arc plasma discharge [6, 7]. Disadvantage of this type of plasma system are: short life of electrodes due to erosion; high temperature non-uniform plasma arc, which makes difficulties to control process temperature; low efficiency for in-flight powder treatment. A continuing need exist for a system by which the rice husk could be effectively convert into absorbent and the generated synthetic gas could be used for powering the system. This objective is achieved by using the thermal heating of rice husk in gas plasma contained atmosphere, precipitating a solid material resulting from pyrolytic reaction; cooling the interaction product: sorbent and synthetic gas; collecting the solid product; and using the synthetic gas for power generation for the system or for direct heating of the reactor by combustion of the synthetic gas.
In a presently preferred form of the invention, the rice husk is introduced into the plasma discharge in a flow of carrier gas. The plasma discharge is generated in a reactor chamber having a longitudinal axis between an inlet end and outlet end, and a plasma gas mixture circulates in the reactor chamber in a reciprocal flow pattern with a zone of substantially axial flow of the mixture in the chamber. The rice husk is preferably fed into the plasma discharge stream in or near this zone at a rate such that the rice husk will be completely carbonized and only solid and gas products will be generate. The plasma flow upstream of the plasma discharge is subjected to rapid cooling, which causes precipitation of solid component of the reaction products in the form of a fine carbon chips. The solid product (sorbent) is collected, but the gas product (synthetic gas) is transported to gas liquation unit for future use in gas turbine for power production. The system of this invention maybe carried out in an apparatus which includes a high frequency plasma torch of either the induction or capacitive type for generating a plasma discharge in oxygen free gas environment. A vortex generator mounted at an inlet of the plasma chamber introduces the plasma gas and creates the reciprocal flow pattern with the zone of axial flow of the plasma gas in the reactor. A quenching unit may take various forms. In a first form the quenching unit injects the coolant fluid into the plasma flow upstream of the plasma discharge. In a second form the quenching unit includes a cold surface disposed for collecting solid reaction product flowing from the outlet end of the reactor. This and other features and advantages of the improved plasma system for practicing the same will be better understood from the following detailed description.